U.S. Pat. No. 3,655,810, European Pat. No. 12 685 and French Pat. No. 2 464 243, describe a process for dimerizing and/or co-dimerizing olefins, for example those having 2 to 4 carbon atoms, such as ethylene, propylene, 1 and 2-butenes, in the presence of a catalyst comprising a mixture of a nickel salt or complex, soluble in hydrocarbons, and of a hydrocarbylaluminum chloride.
In order to industrially develop this process, it is necessary to remove the inorganic part of the catalyst, the presence of which is inconvenient for many uses of the oligomers.
A mere water washing of oligomer hydrocarbons is inefficient, leading to the formation of a gelatinous alumina precipitate and resulting in the formation of chlorinated hydrocarbon derivatives and of a hydrophobic mixture of nickel hydroxide and colloidal metal nickel, which is difficult to remove from the solution. Furthermore, a substantial amount of nickel remains in the hydrocarbon phase as a soluble and stable complex, which decomposes in the reboilers during subsequent fractionations by distillation.
The use of an aqueous solution of alkali metal hydroxide avoids the formation of gelatinous alumina precipitate but does not prevent the formation of chlorinated hydrocarbons.
The treatment of the raw oligomerization product with anhydrous ammonia, followed with water washing, as described in the French Pat. No. 2 114 114, prevents the formation of chlorinated hydrocarbons but not that of a gelatinous alumina precipitate and does not eliminate the nickel compounds soluble in the hydrocarbon phase.
It has also been proposed to treat the oligomerization raw effluent, first with oxygen, then with anhydrous ammonia, and finally with an aqueous solution of alkali metal hydroxide in the presence of oxygen. This results in the destruction of the soluble nickel derivatives in the oligomers and avoids the formation of particles of hydrophobic nickel derivatives. Nickel thus precipitates as insoluble compound which apparently consists of a nickel and aluminum double hydroxide. The presence of this insoluble compound may however impede the process.